The present invention relates to a holographic reconstruction system for the three-dimensional reconstruction of a scene with spatial light modulator means, which carry a holographic code, and with illumination means for illuminating the light modulator means. The present invention chiefly relates to a real-time or near-real-time reconstruction of moving scenes with the help of sequences of video holograms. A hologram signal processor computes video holograms and encodes them on a modulator cell structure of spatial light modulator means which, when illuminated by illumination means which are capable of generating interference, spatially modulate at least one light wave front with holographic information. The modulated light wave front reconstructs the scene through interference of the light waves and thus propagates towards an eye position such that one or multiple observers can watch the reconstructed object light points as a three-dimensional reconstruction of a scene. This means that the reconstructed object light points represent the optical appearance of the scene in a three-dimensional manner in front of the eye positions. In reconstruction systems with low resolution of the modulator cells in the spatial light modulator means and reduced computational load for computing the video holograms, the modulated light wave front is preferably reduced with focussing means to a visibility region of few millimeters up to few centimeters. The reconstruction with a modulated wave front can then not be seen simultaneously by both eyes of an observer. The eyes of an observer can then be served in a time- or space-division multiplexed presentation of different video holograms which differ in parallax.
High-resolution, flat light modulators which have resolutions of several million pixels and which are used as screens in video and TV devices or projectors are for example particularly suited as light modulator means. They achieve the larger light diffraction angles the smaller the distance between the centres of the modulator cells, i.e. the modulator pitch.
A reconstruction system is known from the international publication WO 2004/044659, titled “Video hologram and device for reconstructing video holograms”, which employs for spatial light modulation a liquid crystal display (LCD) panel with conventional resolution, as known from television and video equipment. This reconstruction system comprises focussing means between the illumination means and the light modulator means. With a modulator resolution of a conventional liquid crystal display, which is rather low for video holograms, they allow a holographically reconstructed scene to be made visible in a reconstruction space which stretches between the spatial light modulator and a visibility region at an eye position in a large viewing angle and with great spatial depth at good resolution for at least one observer.
One disadvantage of such light modulators is that despite the relatively small pitch of their modulator cells, they still have a diffraction angle which is too small for an observer to see a reconstruction with both eyes.
The reconstruction system according to the publication WO 2004/044659 thus additionally discloses a possibility for directing and tracking the position of multiple visibility regions. In particular, the reconstruction system realises a mechanical or electronic offset of the light sources laterally to the optical axis of the system using moving mirrors or multiple differently positioned light sources for displacing the light source images which generate the visibility regions for perceiving the reconstruction.
It is disadvantageous that in a large tracking range, substantial aberrations, which occur when light passes through the focussing means, adversely affect the reconstruction of the spatial scene.
In the international publication WO 2006/119920, titled “Device for holographic reconstruction of three-dimensional scenes”, the applicant also discloses a holographic reconstruction system which uses at least one visibility region which is smaller than the modulator surface of the light modulator at an eye position for watching the reconstruction. FIG. 1 shows the functional principle of the prior art system.
An array of light sources LQ1 . . . LQ8 which are capable of generating interference and which are arranged in a matrix and which form a surface-emitting backlight illuminates the modulator surface of a spatial light modulator SLM, and an array of focussing means LA comprises a multitude of imaging elements, e.g. convex lenses, which are adjoined mechanically. Each imaging element of the array of focussing means LA is assigned to multiple light source which are capable of generating interference, so to generate a bundle of illumination units which jointly illuminate the modulator surface, where each illumination unit only covers a sub-region of the modulator surface. The light sources which are capable of generating interference in the illumination units are activated with the help of a switchable modulator matrix SM such that the imaging elements of the array of focussing means image their assigned light source to an eye position. Each illumination unit thus transmits a partial light wave through a sub-region of the modulator surface and, after separate modulation by the individual sub-regions, the partial light waves are superimposed so to form a common visibility region at an eye position EPR.
Directing and tracking the position of the visibility region to changing eye positions is realised by the additional switching modulator matrix SM with modulator cells which can be switched to a transparent state, e.g. by a so-called LCD shutter array. Depending on the current eye position EPR or EPL, which is detected by an eye finder EF, a system controller SC opens for each imaging element of the array of focussing means a point-shaped light exit I or II for the light which is capable of generating interference, which is focused on the eye position by the imaging elements. This generates a pattern of modulator cells which are switched to a transparent mode. In the case of a lateral change of the current eye position, the position of the visibility region will be adjusted in that the system controller SC laterally displaces the pattern of the modulator cells which are switched to the transparent mode accordingly. In the case of an axial change of the eye position, the system controller SC will modify the distances between the modulator cells in the pattern which are switched to the transparent mode. The mentioned publication also discloses the usage of a switchable light source array with discretely controllable point light sources in order to realise the described process of directing and tracking the light wave field.
However, it has shown that the process of directing and tracking the position of the visibility region by adjusting the propagation of the light wave field according to the described solution exhibits several disadvantages, such as aberrations and great light loss.
The applicant also describes in the international publication WO 2006/119760, titled “Projection device and method for the holographic reconstruction of scenes” a holographic projection system which uses a micro display with a diagonal of few centimeters as light modulator. The device comprises imaging means which image the light which is capable of generating interference in a focal plane, so that a visibility region is generated for an eye position. A first imaging means images a video hologram which is encoded on a light modulator on a focussing display screen in a magnified manner. The display screen images a spatial frequency spectrum of the video hologram at an eye position. An optically enlarged reconstruction of the scene is thus visible when looking from the visibility region towards the display screen in a large viewing angle. The visibility region is thus the image of the used diffraction order in the Fourier plane of the video hologram. As in the previously described reconstruction systems, the light modulator can be encoded such that the reconstruction space continues behind the second imaging means.
The projection system described in the publication WO 2006/119760 further comprises in a special embodiment for directing and tracking the position of the visibility region controllable deflection means, which realise mechanical, electric or optical directing and tracking. The deflection means are disposed either near the first imaging means and virtually displace the spatial spectrum like a prism, or near the display screen and realise prism function and, optionally, a lens function, in order to track the visibility region laterally and, optionally, axially.
All reconstruction systems described above use light modulator means with a discrete modulator cell structure and a resolution which is rather low for holographic applications. On the one hand, as is generally known, the discrete modulator cell structure causes a periodic continuation of the holographic reconstruction in other diffraction orders of a diffraction interval, so that the visibility may be impaired. On the other hand, the mentioned pitch of the modulator cell structure results in a relatively small diffraction angle, so that in practice a diffraction order of a few millimeters up to few centimeters is available for an undisturbed visibility region. It thus makes sense to combine such a device with a position detection and tracking module. That module directs with the help of wave tracking means the modulated light waves at the current eye position, adjusts the position of the visibility region according to the eye position and tracks it each time the eye position changes.
Serving as one example, the reconstruction system described in the publication WO 2004/044659 discloses the method of displacing light sources for directing and tracking the position of the visibility region. In particular, the system displaces active light sources in an array of light sources mechanically or electronically laterally to the optical axis of the system.
All tracking systems mentioned above have the disadvantage that in a large tracking range aberrations of the imaging means have a disturbing effect on the reconstruction of the spatial scene. The aberrations occur because the light is transmitted through the imaging means at different angles, depending on the eye position, in order to reconstruct a scene.
Moreover, the light sources need to be positioned mechanically or, if the light source position is controlled electronically, a high spatial resolution of the light source field needs to be provided. In that case, the array of light sources must comprise a multitude of point light sources for each imaging element of the array of imaging means.
A controllable electro-optic cell, a so-called electrowetting cell, is known from the international publication WO 2004/099847, titled “Electrowetting cell”. These cells take advantage of the capillary effect and electrowetting effect in order to modify the surface tension of liquids using electrostatic potential and so to control the optical refraction behaviour. An electrowetting cell basically comprises a capacitor which is filled between the electrodes with a hydrophobic liquid, such as an oil, and water, where one of the electrodes is coated with a hydrophobic material. Without an electric field being applied, the oil covers the coated electrode as a film, and with an electric field being applied, the water displaces the oil film, because the applied field compensates the polarisation of the dipoles in the water surface. The cell can realise electronically controlled optical lenses and prism elements with a surface area of less than one square millimeter.
An autostereoscopic image display device according to the international publication WO 2004/075526, titled “Autostereoscopic display” emits image light points horizontally in a multitude of directions without a tracking device. The image display device has a backlight which emits collimated light which propagates through the image light points of an image representation device towards an array of optical deflection means with dynamically controllable deflection behaviour. The optical elements are in particular electrowetting cells which are used as controllable lenses, and which realise a dynamically adjustable beam controller. In order to avoid the image representation having to be tracked to the current eye position of observers, a system controller frequently modifies with the help of the controllable array of optical deflection means both the exit angle of the light and the image content of the image representation device during each video image period. Thereby, up to one hundred emission directions are served in each video image period using a combination of space-division and time-division multiplexing, said emission directions lying closely side by side horizontally and having the form of image sectors, so that each observer eye sees video images which differ in parallax without the need of tracking. The optical deflection means thus pan the beams which are temporally differently modulated by the image representation device over the multitude of the image sectors which lie closely side by side. The publication does not disclose any technical means which would explain how the system controller can deflect a modulated wave field which is capable of generating interference with the help of the array of optical deflection means.
In contrast to the subject of the present invention, the international publication WO 2004/075526 relates to an autostereoscopic image display device which does not reconstruct object light points in a holographic manner as a three-dimensional arrangement in a viewing space. Instead of the reconstructed object light points, an autostereoscopic image display device displays two-dimensional images in the modulator plane, said two-dimensional images having the form of luminous image points which carry multiple image information for both observer eyes. The cited document does not suggest in any way that light diffraction or light interference have any useful function for image representation. The dynamically adjustable beam controller is designed to deflect bundles of rays with incoherent light in a simple manner and does not make any demands on the conditions for mutual interference of the deflected light beams. The bundles of rays, which lie close to each other, are in particular not able to prevent light of parasitic diffraction orders from entering. Moreover, a non-linear transmission behaviour in the boundary zones of the electrowetting cells would affect the propagation of the modulated light waves which are capable of generating interference, and would substantially disturb the interference behaviour of the reconstruction system and thus the quality of the reconstruction.
The publication does not disclose any technical means which would explain how the system controller can deflect a modulated wave field which is capable of generating interference with the help of the array of optical deflection means and how the effects of parasitic diffraction orders can be circumvented.